(a) Field of the Invention
This invention relates to an apparatus for making gravimetric readings, and more particularly, but not by way of limitation, to a borehole gravity density measuring apparatus for measuring a precise distance between two or more gravity reading positions, and the method of using the same.
(b) Discussion of the Prior Art
Heretofore, gravity meters have been used to find, monitor, and delineate gas reservoirs. The density of a geologic formation can be calculated from the differences between gravity and depth measurements made at two different depth points in a formation, and low densities have been indicative of the presence of porous rock which typically denote gas reservoirs. Most commonly in the past, the depth has been measured from the surface travel of a cable, which is not always an accurate indication of the depth of a tool deep in the well. When a gravity meter is lowered on a cable for a great distance, for example, several thousand feet, the cable is prone to unpredictable degrees of stretch, which are dependent on little understood factors such as cable tension, temperature, pressure, and other downhole conditions which combine to make measurements of the gravity meter depth subject to uncertain errors. Reliable densities have been particularly difficult to obtain for thin beds of less than 15 feet, and multiple passes have often been required to obtain accurate density readings.
In U.S. Pat. No. 4,809,545 to Lyle, a gravity gradient logging tool is disclosed which is received inside a sonde. The sonde has a pair of spaced apart piezoelectric transducers. Each piezoelectric transducer is loaded with a known mass and excited electrically so as to vibrate at a characteristic resonant frequency. The differences in the period of vibration of each transducer is used to determine a measure of gravity gradient along a borehole formation. While this prior art reference uses two different locations within the sonde to determine gravity, it is totally electronic in its operations, has no mechanical moving parts, will produce different signal information, has no means for taking a plurality of gravity meter readings within the sonde's pressure vessel independent of raising and lowering the sonde in the bore hole, and is intended for use while the sonde is moving, rather than stationery, within the borehole.
In U.S. Pat. No. 2,570,659 to Fay, a gravity measuring device using capacitance to measure gravity is described. The device is further directed toward a mechanism for self-levelling a condenser and controlling the temperature of a sonde. A series of related U.S. Pat. Nos. 2,657,581 to Fay et al; 2,953,023 to Goodell; and 3,264,875 to Goodell et al; disclose systems for measuring gravity force as a function of the vibration of a string in the sonde, with the second and third patents disclosing means for releasing a clamp within the system which initiates the vibration with the least amount of distortion or disturbance upon release.
In U.S. Pat. No. 2,618,156 to Boucher, a gravity and density gravimeter is described which utilizes weighted balance beams in a sonde. In U.S. Pat. No. 2,930,138 to La Coste, a mechanism for mounting and levelling a gravity meter is disclosed; and U.S. Pat. No. 3,027,767 to Slack, et al describes an electronic gravity meter which measures the velocity of a falling object in a vacuum in a sonde.
U.S. Pat. No. 4,445,371 to Lautzenhiser describes a mechanism for determining the acceleration of gravity by generating an oscillation and a natural frequency utilizing a magnetic vibrator. The natural frequency being representative of the acceleration of gravity at the location, and generating a signal correlated to oscillation in order to determine the acceleration of gravity.
Also a series of U.S. Pat. Nos. 4,475,386 Fitch et al; 4,596,139 to Gournay; and 4,602,508 Fitch et al relate to borehole gravity meters which utilize pressure transducers which are sensitive to the pressure differential of a fluid within a borehole.
None of the above references found, when taken alone, or in combination, discloses or suggests the use of a gravity meter which is movably suspended within a pressure vessel by a cable, including means, such as a stepper motor, for precisely moving the gravity meter between two positions within the vessel with appropriate electronics for controlling the operation.